Re: Unrolling recursion

[clintp]

One general technique is to use a stack:

seed the stack with the first instance
while (stack has entries) 
     pop off top entry from stack
     process it
     take any potential new work as a result of 
          processing, put it back on the stack

And depth/breadth first recursion can be adjusted by shifting instead of popping at the top of the loop (or when inserting new work).

Of course this only replaces the subroutine stack with your own, but it can be quicker and save memory. (Sometimes it's a lot more obvious too!)

[clintp]

From a /msg request for dragonchild, here's pseudocode to do a directory traversal without recursion.

# Traverse the filesystem
push to stack /
while currentdirectory = pop stack
     open the currentdirectory
     foreach pathname in this directory
           if it's also a directory, push it (full path) 
              on the stack
     close the current directory

As opposed to a recursive solution:

call traverse(/)
function traverse (currentdirectory)
     open currentdirectory
     foreach pathname in this directory
           if it's also a directory, traverse(pathname)
     close currentdirectory

[broquaint]

For those who don't grok pseudocode here's a perl translation for directory traversal without recursion

my @stack = ".";
while(my $cd = pop @stack) {
    opendir(my $dh, $cd) or die("bad dir - $!");
    foreach (readdir($dh)) {
        next if /^\./;
        print $_.$/;
        push @stack, "$cd/$_" if -d "$cd/$_";
    }
}

and with recursion

&traverse(".");
sub traverse {
    my $dir = shift;
    opendir(my $dh, $dir) or die("bad dir - $!");
    foreach (readdir($dh)) {
        next if /^\./;
        print $_.$/;
        &traverse("$dir/$_") if -d "$dir/$_";
    }
}

That's a pretty neat 'trick' which I guess might be how optimising compilers implement some forms of recursion (or something).
HTH

broquaint